1. Field of the Invention
The present invention relates to a sheet switch module used in small electronic devices or the like such as mobile phones.
2. Description of the Related Art
A conventional key switch used in small electronic devices or the like includes, as shown in FIG.5, a circuit board 2, a plurality of switches 10 provided on an upper surface of the circuit board 2, a plurality of light sources 8 mounted on the upper surface of the circuit board 2, and a light-guiding plate 12 disposed to cover the switches 10 (for reference, see Japanese Patent Application Publication No. 2007-53063, in particular FIG. 9).
Each of the switches 10 includes a fixed contact 4 mounted on the upper surface of the circuit board 2 and a movable contact 6 which is disposed to cover the fixed contact 4 and comprises a tact spring. A key top 14 is provided above each of the switches 10. Light emitted from each of the light sources 8 enters the light guiding-plate 12, thereby each key top 14 is illuminated by light emitted from the light guiding plate 12.
In the aforementioned key switch, because the light-guiding plate 12 is merely placed above the switches, a large gap or displacement tends to occur between each switch 10 and the light-guiding plate 12, and the light-guiding plate 12 and each key top 14, and it is difficult to align each switch 10 and each key top 14, because of a displacement is generated therebetween. Consequently, there is a problem that efficiency of light guided from the light source 8 through the light guiding plate 12 to each key top 14 is reduced.
To overcome this, there has been proposed a structure in which a light-guiding sheet 16 is disposed to cover an upper surface of the movable contact 6 and an upper surface of the circuit board 2 and adhered thereto through an adhesive 18 (for reference, see FIG. 1 or the like of above-mentioned Japanese Patent Application Publication No. 2007-53063). In this way, when the light-guiding sheet 16 is closely fitted to each of the switches 10 and so on, and adhered thereto, a displacement between each switch and the corresponding key top is avoided, and a gap between the light-guiding sheet 16 and each switch 10 or light sources 8 is avoided.
However, in the conventional technology disclosed in above-mentioned Japanese Patent Application Publication No. 2007-53063, the light-guiding sheet 16 tends to curve at a boundary between the upper surface of the movable contact 6 and the upper surface of the circuit board 2, light is diffused at the curved portion thereby generating loss of the light, and there is a problem that light-guiding efficiency throughout the entire light-guiding sheet is reduced.